Yep, it's all about genetics. Just like cis girls roll the genetic dice, so do we. (And just like them, there's no guarantees, because sometimes there's throwbacks! My mother is tiny, but my sis and I take after a busty grandmother.) I don't think I believe the "one cup size smaller" theory, but even if it's true, everyone is right that it's still keyed to the hereditary aspect.
I know skinny, tiny cis women with big boobs and big, tall cis women who are small-breasted, so there's just no way to know. 🙂 But I do agree about getting implants too early - doing it 2 years in would be like a cis girl who's 10-13 getting implants (no reputable doctor would do it!).